The present invention relates to films comprising a liquid polarizing suspension dispersed in a polymeric resin. More particularly, it relates to films comprising microdroplets and/or interconnected structure of a liquid polarizing suspension. It also relates to methods for fabricating such films.
A light valve was invented and patented by Edwin H. Land (U.S. Pat. Nos. 1,951,664 and 1,955,923). Land's light valve is a cell comprising two substrates, with at least one of them being transparent. The substrates bear electrodes, usually in the form of a transparent conductive oxide (TCO) coating. The substrates, which form the walls of the cell, are spaced a small distance apart. The cell contains a suspension of small particles in a liquid suspending medium. In the absence of an applied electric field, the particles in the liquid suspension exhibit random Brownian motion, and a beam of light passing into the cell is reflected, transmitted or absorbed depending upon the nature and concentration of the particles and the energy content of the light. When an electric field is applied through the suspension, the particles become aligned parallel to the electric field, and most of the light can pass through the cell. Difficulties with these early light valves included particle settling, particle clustering, color change by heat, reduction of off-state optical density, and degradation by ultraviolet (UV) light, so that suspended-particle light valves have not heretofore been developed into practical commercial devices.
Some of these difficulties, such as particle settling, particle clustering, color change by heat, and reduction of off-state optical density have been solved by R. L. Saxe (U.S. Pat. Nos. 3,756,700; 4,247,175; 4,273,422; 4,407,565; and 4,422,963), F. C. Lowell (U.S. Pat. No. 3,912,365), and R. I. Thompson (U.S. Pat. No. 4,078,856). Here, light valves contained a liquid suspension of solid particles of colloidal size, a polymeric system for helping to disperse the particles in the liquid, and a polymeric stabilizer. Mentioned difficulties were overcome by matching of the densities of suspended particles and suspending medium, and by adding the polymeric stabilizer.
However, with particles in liquid suspension, and when the vertical dimension of a cell is large, hydrostatic pressure of the liquid suspension causes a pressure difference between upper and lower portions of a cell. Resulting net outward forces on the substrates cause bowing and distortion of the substrates into non-planar surfaces. Bowing or distortion in turn leads to non-uniform suspension thickness and non-uniform optical density across the cell. Also, when such light valves are used as windows for buildings, a corresponding inward-bowing effect can result due to wind or difference in pressure between inside and outside a building. Other disadvantages are degradation due to UV-radiation, difficulties in filling or pouring the liquid suspension into a cell, agglomeration of particles in suspension, leaking of the suspension due to failure of a seal, and unequal response time and optical density due to a voltage drop from an edge to the middle of a TCO-substrate.
J. Fergason (U.S. Pat. Nos. 4,435,047; 4,579,423; and 4,616,903) and J. L. West (U.S. Pat. No. 4,685,771) invented films comprising microdroplets of a liquid-crystal material dispersed in a polymeric matrix. In these films, micron-size liquid-crystal droplets are dispersed in and surrounded by polymeric material. In the absence of an applied field, light passing through the film is strongly forward-scattered, giving the film a milky translucent appearance. When an electric field is applied, the liquid-crystal molecules become aligned and the film becomes transparent. Formation of uniform liquid-crystal droplets in a polymeric matrix can be achieved by one of a number of phase separation processes, involving, e.g., polymerization, thermal processing, solvent evaporation, or emulsifying.
The light valves invented by J. Fergason and J. L. West are milky translucent in the absence of an applied voltage. Their light scattering properties detract from their aesthetic appeal and thus limit their applicability to displays and windows. Furthermore, the liquid crystal partially interacts with the polymeric matrix, as a plasticizer and a mixture of several liquid-crystal materials is used. As a result, in the presence of an applied field, the transparent state remains milky because of a difference between the refractive indices of the liquid crystal and the polymeric matrix. Optical use as a light valve is limited to switching functions, between transparent and opaque states. Such a light valve can be used for curtainless windows and the like, but is not applicable for so-called smart windows, where continuous control of transmittance of solar radiation is required. Moreover, since liquid-crystal materials are affected by UV-radiation, a UV-blocking filter or UV-absorbing additive is necessary. Specifically with respect to nematic liquid-crystal materials, their applicability is limited because of temperature sensitivity.